How can I make my string property nullable?

C# 8.0 is published now, so you can make reference types nullable too. For this you have to add

#nullable enable

Feature over your namespace. It is detailed here

For example something like this will work:

#nullable enable
namespace TestCSharpEight
{
  public class Developer
  {
    public string? FullName { get; set; }
    public string? UserName { get; set; }

    public Developer(string fullName)
    {
        FullName = fullName;
        UserName = null;
    }
}}

If you want to globally set nullable property for whole project; you can do it in .csproj file by adding

<Nullable>enable</Nullable>

property to PropertyGroup. It will enable nullablity for all classes in the project.

<Project Sdk="Microsoft.NET.Sdk"> 
  <PropertyGroup> 
    <OutputType>WinExe</OutputType>
     <TargetFramework>net6.0-windows</TargetFramework>
     <Nullable>enable</Nullable>
     <UseWPF>true</UseWPF> 
   </PropertyGroup>

 <!-- Other Settings --> 
 </Project>

Also you can have a look this nice article from John Skeet that explains details.

System.String is a reference type so you don't need to do anything like

Nullable<string>

It already has a null value (the null reference):

string x = null; // No problems here

Strings are nullable in C# anyway because they are reference types. You can just use public string CMName { get; set; } and you'll be able to set it to null.

String is a reference type and always nullable, you don't need to do anything special. Specifying that a type is nullable is necessary only for value types.